You can use a mixture of bleach (sodium hypochlorite) & water, use 20% bleach & 80% water. Wear rubber gloves and wipe the stuff on, leave for 10minutes before rinsing off well with fresh water. You can use the same stuff to rinse out the inside of the tank if you want but be careful because bleach can damage the silicon holding the glass together.
You can wipe the glass down with fresh water before using a single sided razor blade and scrape the stuff of the glass. Then wipe the glass down with fresh water again. Use a squeegy (window wiper thing) to remove any excess water and give a spotless finish.
You can use methylated spirits (95% alcohol) and water to clean the glass with. Use a couple of tablespoons of alcohol per litre of water. Wipe it on, leave for a couple of minutes before scraping it off. Then squeegy for a clean finish.